Linear accelerators of charged particles. How do charged particle accelerators work. Why do we need accelerators of charged particles?

The accelerator of charged particles is a device in which a beam of electrically charged atomic or subatomic particles is created, moving with near-light velocities. The basis of his work is to increase their energy by an electric field and change the trajectory - magnetic.

Why do we need charged particle accelerators?

These devices have found wide application in various fields of science and industry. To date, there are more than 30,000 of them worldwide. For physics, charged-particle accelerators serve as a tool for fundamental studies of the structure of atoms, the nature of nuclear forces, and the properties of nuclei that do not occur in nature. The latter include transuranium and other unstable elements.

With the help of a discharge tube, it became possible to determine the specific charge. Particle accelerators are also used for the production of radioisotopes, industrial radiography, radiation therapy, sterilization of biological materials, and radiocarbon analysis. The largest installations are used in studies of fundamental interactions.

The lifetime of charged particles, which are at rest relative to the accelerator, is smaller than for particles dispersed to velocities close to the speed of light. This confirms the relativity of the time intervals of SRT. For example, at CERN, the muon lifetime was increased by a factor of 29 at a rate of 0.9994c.

This article deals with how the charged particle accelerator, its development, various types and distinctive features are arranged and operating.

Principles of acceleration

Regardless of which accelerators of charged particles are known to you, they all have common elements. First, they all must have an electron source in the case of a television picture tube or electrons, protons and their antiparticles in the case of larger installations. In addition, they all must have electric fields to accelerate particles and magnetic fields to control their trajectory. In addition, the vacuum in the accelerator of charged particles (10 ^-11 mm Hg), i.e., the minimum amount of residual air, is necessary to ensure a long lifetime of the beams. And, finally, all facilities should have the means of recording, counting and measuring accelerated particles.

Generation

Electrons and protons, which are most often used in accelerators, are found in all materials, but first they need to be isolated from them. Electrons, as a rule, are generated in the same way as in a kinescope - in a device called a "gun". It is a cathode (negative electrode) in a vacuum, which is heated to a state where the electrons begin to break away from the atoms. Negatively charged particles are attracted to the anode (positive electrode) and pass through the outlet. The gun itself is also the simplest accelerator, since the electrons move under the action of the electric field. The voltage between the cathode and the anode, as a rule, is within 50-150 kV.

In addition to electrons, all materials contain protons, but only single nuclei of hydrogen atoms consist of single protons. Therefore, the source of particles for proton accelerators is hydrogen gas. In this case, the gas is ionized and the protons exit through the hole. In large accelerators, protons are often formed in the form of negative hydrogen ions. They are atoms with an additional electron, which are the product of the ionization of a diatomic gas. With negatively charged hydrogen ions at the initial stages it is easier to work. Then they are passed through a thin foil, which deprives them of electrons before the final stage of acceleration.

Overclocking

How do charged particle accelerators work? A key feature of any of these is the electric field. The simplest example is a uniform static field between positive and negative electrical potentials, similar to the one that exists between the terminals of an electrical battery. In such a field, an electron carrying a negative charge is subject to the action of a force that directs it to a positive potential. It accelerates it, and if there is nothing to prevent it, its speed and energy increase. Electrons moving toward a positive potential along a wire or even in air collide with atoms and lose energy, but if they are in a vacuum, they are accelerated as they approach the anode.

The voltage between the initial and final positions of the electron determines the energy it has acquired. When moving through a potential difference of 1 V, it is 1 electron-volt (eV). This is equivalent to 1.6 × 10 ^-19 joules. The energy of a flying mosquito is a trillion times larger. In the kinescope, electrons are accelerated by a voltage of more than 10 kV. Many accelerators reach much higher energies, measured by mega, giga, and teraelectron volts.

Varieties

Some of the earliest types of charged particle accelerators, such as the voltage multiplier and the Van de Graaff generator, used constant electric fields created by potentials of up to a million volts. With such high voltages it is not easy to work. A more practical alternative is the repeated action of weak electric fields created by low potentials. This principle is used in two types of modern accelerators - linear and cyclic (mainly in cyclotrons and synchrotrons). Linear accelerators of charged particles, briefly, skip them once through a sequence of accelerating fields, while in cyclic they repeatedly move along a circular trajectory through relatively small electric fields. In both cases, the final energy of the particles depends on the total action of the fields, so many small "jerks" are added together to give the cumulative effect of one large.

The repeated structure of a linear accelerator for the creation of electric fields naturally implies the use of alternating voltage, not constant voltage. Positively charged particles are accelerated to a negative potential and get a new push if they pass by the positive one. In practice, the voltage should change very quickly. For example, at an energy of 1 MeV, the proton moves at very high speeds, making up 0.46 times the speed of light, passing 1.4 m in 0.01 ms. This means that in a repeating structure of several meters in length, the electric fields must change direction with a frequency of at least 100 MHz. Linear and cyclic accelerators of charged particles, as a rule, disperse them with the help of alternating electric fields with frequency from 100 to 3000 MHz, i.e., ranging from radio waves to microwaves.

Electromagnetic wave is a combination of alternating electric and magnetic fields, oscillating perpendicular to each other. The key point of the accelerator is the tuning of the wave so that when the particle arrives the electric field is directed in accordance with the acceleration vector. This can be done with the help of a standing wave - a combination of waves moving in opposite directions in a closed space, like sound waves in the organ tube. An alternative option for very rapidly moving electrons, whose velocity is approaching the speed of light, is a traveling wave.

Autophasing

An important effect in acceleration in an alternating electric field is "autophasing". In one oscillation cycle, the alternating field passes from zero through the maximum value again to zero, drops to a minimum, and rises to zero. Thus, it passes twice through the value necessary for acceleration. If a particle whose speed increases, arrives too early, then it will not have a field of sufficient strength, and the push will be weak. When it reaches the next section, it will be late and will experience a stronger impact. As a result, autophasing will occur, the particles will be in phase with the field in each accelerating region. Another effect will be their grouping in time with the formation of clots, rather than a continuous flow.

Beam direction

An important role in how the accelerator of charged particles is arranged and operated is played by magnetic fields, since they can change the direction of their motion. This means that they can be used to "bend" the beams along a circular path so that they travel several times through the same accelerating section. In the simplest case, a force acting perpendicular to both the displacement vector and the field acts on a charged particle moving at right angles to the direction of a uniform magnetic field. This causes the beam to move along a circular path perpendicular to the field, until it leaves the area of its action, or another force starts acting on it. This effect is used in cyclic accelerators, such as cyclotron and synchrotron. In a cyclotron, a constant magnetic field creates a constant magnetic field. Particles as they grow their energy move spirally outward, accelerating with each turn. In the synchrotron, the bunches move around the ring with a constant radius, and the field created by the electromagnets around the ring increases as the particles accelerate. Magnets that provide a "bend" are dipoles with the north and south poles bent in the form of a horseshoe in such a way that the beam can pass between them.

The second important function of electromagnets is the concentration of beams, so that they are as narrow and intense as possible. The simplest form of a focusing magnet is with four poles (two north and two south) located opposite each other. They push the particles toward the center in one direction, but allow them to propagate in a perpendicular direction. Quadrupole magnets focus the beam horizontally, allowing it to exit from the focus vertically. For this they must be used in pairs. For more accurate focusing, more complex magnets with a large number of poles (6 and 8) are also used.

As the energy of the particles increases, the strength of the magnetic field that guides them increases. This keeps the beam on the same path. The bunch is introduced into the ring and accelerated to the required energy before it is withdrawn and used in experiments. The withdrawal is achieved by electromagnets, which are switched on to expel the particles from the synchrotron ring.

Collision

Particle accelerators used in medicine and industry generally produce a beam for a particular purpose, for example, for radiation therapy or ion implantation. This means that the particles are used once. For many years, the same was true for accelerators used in basic research. But in the 1970s, rings were developed in which two beams circulate in opposite directions and collide all over the contour. The main advantage of such installations is that, in a head-on collision, the energy of the particles passes directly into the energy of interaction between them. This contrasts with what happens when a beam collides with a material at rest: in this case, most of the energy goes to bring the target material into motion, in accordance with the principle of conservation of momentum.

Some machines with colliding beams are built with two rings intersecting in two or more places in which particles of the same type circulate in opposite directions. Colliders with particles and antiparticles are more common. The antiparticle has the opposite charge of the particle bound to it. For example, the positron is positively charged, and the electron is negative. This means that the field that accelerates the electron slows the positron moving in the same direction. But if the latter moves in the opposite direction, it will accelerate. Similarly, an electron moving through a magnetic field will bend to the left, and the positron to the right. But if the positron moves to meet, its path will continue to deviate to the right, but along the same curve as the electron. Together, this means that these particles can move along the synchrotron ring due to the same magnets and be accelerated by the same electric fields in opposite directions. By this principle, many powerful colliders are created on colliding beams, since only one accelerator ring is required.

The beam in the synchrotron does not move continuously, but is combined into "clots". They can have several centimeters in length and a tenth of a millimeter in diameter, and contain about 10 ¹² particles. This is a small density, because in a substance of similar dimensions contains about 10 ²³ atoms. Therefore, when the beams intersect with colliding beams, there is only a small probability that the particles will interact with each other. In practice, the clots continue to move along the ring and meet again. A deep vacuum in a charged particle accelerator (10 ^-11 mm Hg) is necessary to allow the particles to circulate for many hours without colliding with air molecules. Therefore, rings are also called cumulative rings, since the beams are actually stored in them for several hours.

check in

Accelerators of charged particles in the majority can register an event at hit of particles in the target or in other bundle moving in an opposite direction. In the television picture tube, electrons from the gun are struck in the phosphor on the inner surface of the screen and emit light, which thus recreates the transmitted image. In accelerators, these specialized detectors react to scattered particles, but they are usually designed to generate electrical signals that can be converted into computer data and analyzed using computer programs. Only charged elements create electrical signals passing through the material, for example, by exciting or ionizing atoms, and can be detected directly. Neutral particles, such as neutrons or photons, can be detected indirectly through the behavior of charged particles that are driven by them.

There are many specialized detectors. Some of them, such as the Geiger counter, simply count the particles, while others are used, for example, to record tracks, measure speed or quantity of energy. Modern size detectors and technologies range from small charge-coupled devices to large gas-filled cameras with wires that detect ionized traces created by charged particles.

History

Accelerators of charged particles were mainly developed for studies of the properties of atomic nuclei and elementary particles. Since the discovery of the British physicist Ernest Rutherford in 1919, the reactions of the nucleus of nitrogen and alpha particles, all research in nuclear physics until 1932 was conducted with helium nuclei released as a result of the decay of natural radioactive elements. Natural alpha particles have a kinetic energy of 8 MeV, but Rutherford believed that to observe the decay of heavy nuclei it is necessary to artificially accelerate them to even greater values. At that time it seemed difficult. However, the calculation made in 1928 by Georgy Gamow (at the University of Göttingen, Germany) showed that ions with much lower energies can be used, and this stimulated attempts to construct a facility that provided a beam sufficient for nuclear research.

Other events of this period demonstrated the principles by which accelerators of charged particles are built to this day. The first successful experiments with artificially accelerated ions were made by Cockcroft and Walton in 1932 at Cambridge University. Using a voltage multiplier, they accelerated the protons to 710 keV and showed that the latter react with the lithium nucleus to form two alpha particles. By 1931, at the University of Princeton in New Jersey, Robert Van de Graaf built the first belt electrostatic generator of high potential. Cokroft-Walton voltage multipliers and Van de Graaff generators are still used as energy sources for accelerators.

The principle of a linear resonance accelerator was demonstrated by Rolf Wideröe in 1928. At the Rhine-Westphalian Technical University in Aachen, Germany, he used a high alternating voltage to accelerate sodium and potassium ions to energies twice that reported to them. In 1931, in the United States, Ernest Lawrence and his assistant David Sloan of the University of California, Berkeley, used high-frequency fields to accelerate mercury ions to energies exceeding 1.2 MeV. This work was supplemented by the accelerator of heavy charged particles Wideröe, but ion beams were not useful in nuclear research.

A magnetic resonance accelerator, or cyclotron, was conceived by Lawrence as a modification of the Wideröe installation. A student of Lawrence Livingston demonstrated the cyclotron principle in 1931, producing ions with an energy of 80 keV. In 1932, Lawrence and Livingston announced the acceleration of protons to more than 1 MeV. Later in the 1930s, the cyclotron energy reached about 25 MeV, and the Van de Graaff generators about 4 MeV. In 1940 Donald Kerst, using the results of careful calculations of the orbit to the design of magnets, built the first betatron at the University of Illinois, a magnetic induction electron accelerator.

Modern physics: accelerators of charged particles

After the Second World War, the science of accelerating particles to high energies has made rapid progress. He started Edwin Macmillan in Berkeley and Vladimir Veksler in Moscow. In 1945, they both independently described the principle of phase stability. This concept offers the means of maintaining stable orbits of particles in a cyclic accelerator, which removed the restriction on the energy of protons and allowed the creation of magnetic resonance accelerators (synchrotrons) for electrons. Autophasing, the implementation of the principle of phase stability, was confirmed after the construction of a small synchrocyclotron at the University of California and synchrotron in England. Soon after this, the first proton linear resonant accelerator was created. This principle is used in all large proton synchrotrons built since then.

In 1947, William Hansen, at Stanford University in California, built the first linear electron accelerator on a traveling wave, using microwave technology, which was developed for radar during the Second World War.

Progress in research was made possible by increasing the energy of protons, which led to the construction of ever larger accelerators. This trend was stopped by the high cost of making huge ring magnets. The largest weighs about 40,000 tons. Methods for increasing energy without increasing the dimensions of machines were demonstrated in 1952 by Livingston, Courant and Snyder in the technique of alternating focusing (sometimes called strong focusing). Synchrotrons working on this principle use magnets 100 times smaller than before. Such focusing is used in all modern synchrotrons.

In 1956 Kerst realized that if two sets of particles are held in intersecting orbits, one can observe their collisions. The application of this idea required the accumulation of accelerated beams in cycles called cumulative beams. This technology has made it possible to achieve the maximum interaction energy of the particles.